Hi @keithzikai.chew!
The difference has to do with how we display evidence, as you can read in this other Community post: Score values from Disease->Target vs. Target->Disease
Essentially, if you search for diseases associated with a target (your first use case), the association score displayed is calculated using only direct evidence, i.e. in this case, evidence that links PSEN1 to Alzheimer’s disease specifically.
If you search for targets associated with a disease (your second use case), the association score calculation includes both direct and indirect evidence: evidence that links PSEN1 to Alzheimer’s disease and any of its child terms in our ontology such as Early-onset autosomal dominant Alzheimer disease (screenshot below from the Alzheimer’s disease profile page).
This indirect evidence is also displayed in the evidence page for Alzheimer’s disease and PSEN1, see example below, where the ClinVar widget displays evidence for early-onset autosomal dominant Alzheimer disease.
You can also read more about this in our documentation: Target - disease associations - Open Targets Platform Documentation